Welcome Guest! If you are already a member of the BMW MOA, please log in to the forum in the upper right hand corner of this page. Check "Remember Me?" if you wish to stay logged in.

We hope you enjoy the excellent technical knowledge, event information and discussions that the BMWMOA forum provides.
Why not take the time to join the club, so you can enjoy posting on
the forum, the club magazine, and all of the discounts and benefits the BMWMOA offers?Want to read the MOA monthly magazine for free? Take a 3-month test ride of the magazine; check here for details.

If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You will need to join the MOA before you can post: click this register link to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.

NOTE. Some content will be hidden from you. If you want to view all content, you must register for the forum if you are not a member, or if a member, you must be logged in.

The acceptable range of combustible mixture is much wider than the 4% number suggested above, as poor carburetors and automatic chokes have proven for many years. According to Marks Engineering Handbook, it is over 2:1 ( i. e. 7:1 to ~19:1 A/F ratio). In other words, in our case, if the L-R airflow inequality exceeds ~2:1, there will be a right side miss - and I contend that is the source of the surge.

This approach completely eliminated the surge by carefully adjusting the intake valve clearances, thoroughly cleaning the throttle bodies and BBS ports, and resetting the throttle stops & big brass screws (BBSs) to equal positions. It was not necessary to disable or change any of the processor controlled mixture functions at all.

In summary, surge in my case was caused by an imbalance in the throttle stop screw /BBS settings created by throttle body contamination.

Surging is an outcome that can have many causes. The description in the prior post is a good description of one half of the problem of light-load power imbalance. I will be the first to acknowledge that L/R air balance is an important part of getting the L/R cylinders to be power balanced. But it is only half the story, and it is the half we most focus on because we can measure it (intake manifold vacuum balance) and adjust it (BBS, throttle stops, valve clearances).

But the other half of the equation is fuel balance. This is easy to ignore because there is no way to easily measure it, and nothing to adjust. And it is wrong to believe that because the injector pulses are equal, that the amount of fuel or the quality of its atomization is equal.

At the speeds where surging is often experienced, throttle angles are low leading easily to air imbalance. But the injector pulse widths are also small, on the order of 2/1000's of a second. Of that 2mS, half of the time is the time to open the injector and the time to close the injector. So 1 mS is Open/Close and 1mS is spraying fuel. Of the 1 mS when the injector is spraying fuel, there can be different rates and different atomization pattern due to slight fouling and orifice sizes. In addition, as injectors age the open/close, or "dead time" as it's known, changes. This can easily lead to significant differences in fuel L/R and therefore power.

The notion in the prior post that combustion can take place between 7:1 and 19:1 is true, but wrong-minded. If the difference between cylinders was merely (let alone 7:1 or 19:1) 13.2:1 (roughly best power mixture) and 16.4:1 (roughly best economy), a twin cylinder motorcycle would be unrideable. But that gets to my real point which is why adding 4-8% more fuel that a narrowband sensor dictates makes such a significant difference.

In simple terms, you can get the power between cylinders to be close to equal by air balancing alone, IF there is enough fuel in the mixture to consume all the available oxygen. Once there is enough fuel to consume all the O2, then a bit more fuel in one cylinder doesn't effect the power balance L/R. The other thing to do would be to get perfectly equal fuel side to side and then add enough air so that all the fuel was consumed.

There are no adjustments to make fuel equal side to side so we have to balance air side to side and then, for a really smooth running motorcycle, add enough fuel so that all the O2 is consumed. The stock mixture is 14.7:1 and it turns out you only need to add 4-8% more fuel (given typical injector mismatch) to consume all the unburned air. So although 4% doesn't sound like much, it very often does the job. And the best part, of the 4% fuel that gets added (or 8% if you like) half of it is converted to power so not much is wasted.

The last consideration is the O2 sensor itself. The prior post mentions that there is only a TPS on one of the two cylinders. It should be noted that there is only one O2 sensor for both cylinders. Therefore if you perfectly balance the air but the L/R fuel is imbalanced by 6%, one cylinder is running at 14.4:1 and the other is running at 15.0:1 this too is a large imbalance. The greater the injector mismatch, the more additional fuel you need to add, hence my estimate of a 4-8% range.

Interested to hear how you made sure the throttle stops and big brass screws were set to exact equal settings.

I do not have a flow bench setup. I cleaned the throttle bodies & BBS ports with a Q-tip & carb Cleaner. The BBSs were set out 1 1/2 turns from closed. The throttle stops were set using a flashlight from the back side to get just the first indication of opening. Not very scientific but I was especially careful that they both looked the same.

Someone else has advocated a 1/16 turn on the TPS screws from initial contact. Probably about what I had. I then set the TPS to .385 V when the LH throttle was against the new stop setting.

After re-assembling everything, this was all amazingly close to the needed final setting. Maybe I was lucky. I used an oil based U-tube to do a final tweak of the BBSs and to adjust the throttle cables etc.

A lot happens in the first few degrees of throttle opening. The processor has to convert that voltage (plus other info) into an airflow calculation that can be used to update the injector pulse length map. How much depends on the settings of the BBSs vs the throttle stops. Obviously the algorithm must be quite adaptable to a wide range of air flows as the scaling of airflow mass vs millivolts of TPS has to be grossly non-linear as well as wildly different depending on the balance of BBSs setting vs TPS stop setting. Hopefully the slope of this relationship has to be within say a 2:1 ratio for the two sides to prevent a non-combustible mixture error & stumble.

I would like to create a flow bench if someone had a spare throttle body for tests. From it we could establish the relationship between turns of BBS vs turns of throttle backstop to deliver equivalent idle air flow rates.

I did not go after the injectors initially as I discounted contamination since the surge problem for my bike seemed to be come on slowly.

The notion in the prior post that combustion can take place between 7:1 and 19:1 is true, but wrong-minded. If the difference between cylinders was merely (let alone 7:1 or 19:1) 13.2:1 (roughly best power mixture) and 16.4:1 (roughly best economy), a twin cylinder motorcycle would be unrideable. But that gets to my real point which is why adding 4-8% more fuel that a narrowband sensor dictates makes such a significant difference.

I don't think power output vs mixture is that critical - especially in a minimum power situation. More critical is what the map optimization does to both cylinders when only one cylinder is dumping excess oxygen into the converter.

I don't think power output vs mixture is that critical - especially in a minimum power situation. More critical is what the map optimization does to both cylinders when only one cylinder is dumping excess oxygen into the converter.

I gotta study & think about the rest of your post
NRP

At every power level, fuel and air play equal roles. They way to take the fuel imbalance out of the equation is either with very good matching of injectors, individual O2 sensors like the R1200 or on our R1100 and R1150 motocycles, add just enough fuel so that all the oxygen is consumed.

The guys at Gami who make precision matched injectors for piston-engine aircraft disagree. They believe, have measured, and produce matched injectors. Their claim is that most "driveability" issues with fuel injected engines have to do with the torque pulses created by power/AFR imbalances between the cylinders. In fact, their view is that lean-misfire is not the cause of the rough-running caused at low fuel flows (lean mixtures).

To prove their point, they have produced custom fueling arrangements that take into account intake fuel scavenging from adjacent cylinders in common manifold engines. Once they get the air and fuel balanced, they can run much leaner mixtures at the same power level.

Just as further food for thought, first this from an article from GAMI's web site:

Why do variations in fuel/air ratios cause engines to run rough on the lean side of peak but not on the rich side of peak?

The answer is really pretty simple. But it has nothing to do with what you probably have been taught. Most pilots are taught that engines run rough on the lean side of peak due to some mysterious "lean misfire". Nonsense.

The engines run rough because on the lean side of peak, most engines do not have uniform cylinder-to-cylinder horsepower output. This cylinder-to-cylinder horsepower imbalance causes vibration, which has been often and wrongly characterized as lean misfire. We know [that] if one does precisely balance the fuel/air ratios of all of the cylinders, one can lean the engine to more than 100 degrees lean of peak - even 120 to 140F lean of peak ... (100 degrees lean of peak is much leaner than our Oilheads, which run at about peak EGT, operate)

Second, just this past week wjg04oh (who is now running and riding with an LC-1) realized that somewhere around 4000 RPM in 1st, 2nd and 3rd gear, his motorcycle was open loop and much leaner than expected. How lean? Roughly 4% leaner than his Lambda setpoint. I reran his test on my motorcycle and found the same conditions and noticed that AFR changed quickly with small changes of throttle input (read the last 4 or 5 posts here). If our bikes had been running the stock setpoint (14.7:1) our measured AFRs would have been 15.3:1 (average of the two cylinders)--one will be even leaner. Referring to the Gami comments above, these conditions would certainly be in the "rough running" (or surging) territory.

I think the more of us that run with LC-1s connected, the more we will understand the combustion conditions that lead to "rough running".

I don't think power output vs mixture is that critical - especially in a minimum power situation. More critical is what the map optimization does to both cylinders when only one cylinder is dumping excess oxygen into the converter.

I gotta study & think about the rest of your post
NRP

Wait a minute! - I should have said what happens when one cylinder dumps unburned fuel into the converter. This would make both cylinders to quickly be remapped leaner - & probably more stumble.

At any rate, The mass flow to each cylinder vs the milli-volts out of the sole TPS has to be within a fairly small range - whatever that is.

Q - At a typical idle (i. e. against the throttle stops), what is the ratio of butterfly opening area versus BBS metering port area? I frankly don't expect anyone to know this, but that's what I'd like to find out without taking my R1100 down.

Another thought - since we are dealing with the very small angle of TPS motion, could it be that the TPS pot linearity at small angles has something to do with surging too? Film pots do wear, of course and are not particularly durable.

Wait a minute! - I should have said what happens when one cylinder dumps unburned fuel into the converter. This would make both cylinders to quickly be remapped leaner - & probably more stumble..

It would by more apt to say that the Motronic adjusts the Closed Loop mixture to bracket the lambda value of the O2 sensor. So if you lower the O2 sensor's lambda by, for example, 6% the the Motronic will adjust to that 6% richer mixture. You give the Motronic the new goal by installing the richer O2 sensor and the motronic does all the work.

Originally Posted by nrpetersen

At any rate, The mass flow to each cylinder vs the milli-volts out of the sole TPS has to be within a fairly small range - whatever that is..

My tests indicate that it resolves a smallest step of about 0.32 degrees. When cold the Motronic adds enough extra fuel that accuracy doesn't seem to be an issue. When warmed up, closed loop learns exactly how much fuel is needed.

Originally Posted by nrpetersen

Q - At a typical idle (i. e. against the throttle stops), what is the ratio of butterfly opening area versus BBS metering port area? I frankly don't expect anyone to know this, but that's what I'd like to find out without taking my R1100 down..

I don't think I've read any test of it. But we get idle air and 2000 rpm air (vacuum) pretty close. Just switched from twinmax to harmonizer today's. what a good upgrade.

Originally Posted by nrpetersen

Another thought - since we are dealing with the very small angle of TPS motion, could it be that the TPS pot linearity at small angles has something to do with surging too? Film pots do wear, of course and are not particularly durable.

Two things: The TPS pot has two wipers. One goes from 0 to 23 degrees the other covers 20 to 81 degrees. Once the throttle is steady, closed loop learns the small errors and saves them for open loop fueling too.

In summary,

I think that air balance and fuel balance are the two principle issues. The air balance can be adjusted, but the fuel injector imbalances can't. So you got two options: clean and match the injectors or richen the mixture a bit.

I think that air balance and fuel balance are the two principle issues. The air balance can be adjusted, but the fuel injector imbalances can't. So you got two options: clean and match the injectors or richen the mixture a bit.

Why not balance the air measurement system? I think we have to quantify the BBS vs throttle stop screw tradeoffs first. Your data seems to me to be based on assumptions that the air supply system is essentially equal on the two sides. I don't think it is.

Why not balance the air measurement system? I think we have to quantify the BBS vs throttle stop screw tradeoffs first. Your data seems to me to be based on assumptions that the air supply system is essentially equal on the two sides. I don't think it is.

Again the call - does anyone have a spare throttle body?

Of course you would balance the air intake, it is one of two necessary step to getting equal power side to side. Several things are related to the Volumetric Efficiency (VE) which is actually what you hope to balance. Just because the intake manifold vacuum is equal does not guarantee that VE is equal. VE has to do with Intake Manifold dynamics, throttle body/throttle plate/BBS) flow, intake valve/combustion chamber/exhaust value timing and performance, exhaust pressure at the moment of exhaust valve closure. Exhaust pressure is interesting as the shape of the exhaust plumbing is different between the two sides. All that said, based on the smoothness at many loads and RPMs, the VE vs RPM/TPS on my motorcycle seems pretty balanced.

Once you have done that, add enough fuel, a few percent, so that you move to the point on fuel vs power curve to be nearer best power. Once you do that, inequalities in fuel flow are much less significant. Looking at the chart below (the thick red vertical line is about where our bikes run, stoic or peak EGT), second set of curves up from the bottom, you want to be nearer point #2 than point #3. At point # two, changes in fuel flow don't change the power outpout. Nearer point 3, the power changes a lot with AFR and it gets worse, the more to the right (leaner) you go.

If you look at the end of my Wideband O2 thread, wsj04oh has noticed that at small throttle angles and rpms above 3500 RPM that the Motronic is Open Loop even with steady throttle. He has also noticed that the fueling is leaner there than expected. This is also an area where his bike was prone to surge before he added his LC-1. Because he is riding and observing AFR, not just inferring what may or may not be happening, we have another data point suggesting that rough running is related to leanness.

I agree. I have 170K on my 94 RS, still on the original injectors, and I have never had or developed a surging problem. But, I have been using Chevron Techron fuel cleaner (the only one I feel really works well) for well over ten years. I go through at least two bottles of it per season. Never had a fuel system related problem on my RS.

I agree. I have 170K on my 94 RS, still on the original injectors, and I have never had or developed a surging problem. But, I have been using Chevron Techron fuel cleaner (the only one I feel really works well) for well over ten years. I go through at least two bottles of it per season. Never had a fuel system related problem on my RS.

Not really surprising that yours is fine. I think it is a situation where some do and some don't surge, exactly what you would expect in a stack-up-of-tolerences of air and fuel.

There are some other benefits of running a few percent more fuel. The biggest of which, in my mind, is the gain in torque between 2000 and 3500 RPM. First let me say that I think that you should only add the fuel you need 4-6% to improve driveability. That said, I installed a WB O2 that I'm evaluating and although it is set at lambda of 0.94 (6% more fuel), it has some issues and the lambda measured is 0.91 (about 9% more fuel). I don't have any intention of running my bike at 0.91 but, wow, does it run nicely.

I agree. I have 170K on my 94 RS, still on the original injectors, and I have never had or developed a surging problem.

ANDYVH - What is your bike's present configuration? What sort of throttle body history does it have?

Assuming it is stock (which it sounds like few are), I wonder if perhaps the nature of growing parameter discrepancy between air flow calculations on the two sides is completely random. If so, maybe on half the oilheads, if the right cylinder (the one w/o a TPS and independent oxygen sensor) happens to evolve rich (not lean) if then they do not develop a surge/stumble?

Of course you would balance the air intake, it is one of two necessary step to getting equal power side to side. Several things are related to the Volumetric Efficiency (VE) which is actually what you hope to balance. Just because the intake manifold vacuum is equal does not guarantee that VE is equal. VE has to do with Intake Manifold dynamics, throttle body/throttle plate/BBS) flow, intake valve/combustion chamber/exhaust value timing and performance, exhaust pressure at the moment of exhaust valve closure. Exhaust pressure is interesting as the shape of the exhaust plumbing is different between the two sides. All that said, based on the smoothness at many loads and RPMs, the VE vs RPM/TPS on my motorcycle seems pretty balanced.

Once you have done that, add enough fuel, a few percent, so that you move to the point on fuel vs power curve to be nearer best power. Once you do that, inequalities in fuel flow are much less significant. Looking at the chart below (the thick red vertical line is about where our bikes run, stoic or peak EGT), second set of curves up from the bottom, you want to be nearer point #2 than point #3. At point # two, changes in fuel flow don't change the power output. Nearer point 3, the power changes a lot with AFR and it gets worse, the more to the right (leaner) you go.

If you look at the end of my Wideband O2 thread, wsj04oh has noticed that at small throttle angles and rpms above 3500 RPM that the Motronic is Open Loop even with steady throttle. He has also noticed that the fueling is leaner there than expected. This is also an area where his bike was prone to surge before he added his LC-1. Because he is riding and observing AFR, not just inferring what may or may not be happening, we have another data point suggesting that rough running is related to leanness.

If I read you right, your contention is that overall leanness is the cause of surging & that shifting the target Lambda a few percent is the fix. If so, it would seem like both cylinders are participating in the instability via the closed loop mode optimization.

My contention is that only one cylinder is dropping out during our surges, and that stable operation off idle should be achievable with about 14:1 A/F ratio - if it could be made similar on both cylinders.

Does Volumetric Efficiency have a formal definition?

Our Lycoming powered 172M could go a couple hundred degrees lean of peak without getting into any serious stumble. Of course that with a carb and single EGT probe, but it was at ~55% power. I think here we are dealing with misfire in a near over run situation which could be much different from aircraft engine applications around cruise power.

Thank you for bearing down on the points I made earlier and testing them.

Originally Posted by nrpetersen

If I read you right, your contention is that overall leanness is the cause of surging & that shifting the target Lambda a few percent is the fix. If so, it would seem like both cylinders are participating in the instability via the closed loop mode optimization.

Not what I'm contending. Let me go over it again.

I think there are a lot of factors that can contribute to driveability issues in lean-fueled vehicles. Based on a year and a half of data recording and research, left/right power imbalances are
one cause.

For the power to be balanced, there must be equal VE (volume of air and oxygen) in each cylinder and their must be equal air to fuel ratios left to right.

If the left/right AFRs are unequal, it is well known from experience and experiment that the leaner you are relative to the Best Power Mixture, the more susceptible an engine is to roughness, stumbling and vibration (or one form of surging).

No matter how precisely you balance VE, if there is a fuel imbalance and the overall mixture is lean, you are likely to feel it.

You can make an engine much less susceptible to unequal fuel delivery if you balance VE, and richen the mixture toward Best Power.

Originally Posted by nrpetersen

My contention is that only one cylinder is dropping out during our surges, and that stable operation off idle should be achievable with about 14:1 A/F ratio - if it could be made similar on both cylinders.

At an AFR of 14:1 you are about 5% richer than stoic and nearer Best Power Mixture. If you balance air you will and richen to 14:1, you will get a smooth running engine. I would agree with that.

Looking at the hundred or so hours of test data that I have, there is no data showing one cylinder dropping out or misfiring. You should set up some equipment, measure this and show the data.

Originally Posted by nrpetersen

Does Volumetric Efficiency have a formal definition?

Yes, you can easily find it by Googling: Volumetric Efficiency

Originally Posted by nrpetersen

Our Lycoming powered 172M could go a couple hundred degrees lean of peak without getting into any serious stumble. Of course that with a carb and single EGT probe, but it was at ~55% power. I think here we are dealing with misfire in a near over run situation which could be much different from aircraft engine applications around cruise power.

My Continental powered Beech A36 Bonanza could go somewhere in the vicinity of 100 degrees lean of peak and not stumble. But you could lean it to the point where there was no mis-fire but the type of roughness mentioned by Gami in their articles.

As I mentioned earlier, wjg04oh has taken data with his LC-1 that shows there are regions of steady throttle operation (e.g. 3rd gear, 4000 RPM, 7 degrees TPS) where the Motronic is Open Loop and quite lean. I believe this is a candidate for further experiments.

It seems to me that you're trying to convince yourself that if the air can be balanced, that fueling differences don't matter much. That is only true near Best Power Mixture. Lean of Peak EGT or in our case Leaner than Stoic (14.7:1 for gasoline) our boxers, and many other engines, are susceptible to rough running.

I have lots of measurements at this point that show if you balance air (VE) and richen the mixture 4-6%, you get quite a smooth engine. And you get a secondary benefit of a nice boost in torque in the 2000-3500 RPM range. Give it a try.

Roger - I only started to wade thru the 10+ pages of plots etc in yr earlier posts but couldn't yet see where you can identify which cylinder is missing - (or if they both are). Can you direct me?
THX
Niel